3D cross-bar nonvolatile memory

ABSTRACT

Semiconductor structures and methods for crystalline junctionless transistors used in nonvolatile memory arrays are introduced. Various embodiments in accordance with this disclosure provide a method of fabricating a monolithic 3D cross-bar nonvolatile memory array with low thermal budget. The method incorporates crystalline junctionless transistors into nonvolatile memory structures by transferring a layer of doped crystalline semiconductor material from a seed wafer to form the source, drain, and connecting channel of the junctionless transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/253,189, titled “3D Cross-Bar Nonvolatile Memory,” filed on Aug. 31,2016, and a continuation of U.S. application Ser. No. 15/489,196, titled“3D Cross-Bar Nonvolatile Memory,” filed on Apr. 17, 2017, which areincorporated by reference herein in their entireties.

BACKGROUND

Nonvolatile memory is often used in various devices, such as computers.Nonvolatile memory is a type of memory storage that can retain data evenwhile it is not powered on. Nonvolatile memory may be electricallyaddressed. Examples of electrically addressed nonvolatile memory includeflash memory, electrically programmable read-only memory (EPROM), andelectrically erasable programmable read-only memory (EEPROM).Functionality of nonvolatile memory includes having informationprogrammed into it, having information read from it, and/or havinginformation erased from it.

Nonvolatile memory circuits often include electrical components such as,for example, diodes, capacitors, and resistors, each of which may becombined with transistors to form an electrical circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the common practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofillustration and discussion.

FIG. 1 is an isometric view of a junctionless transistor structure, inaccordance with some embodiments.

FIG. 2 is a cross-sectional view of a n-type silicon junctionlesstransistor, in accordance with this disclosure.

FIG. 3 is a cross-sectional view of a p-type silicon junctionlesstransistor, in accordance with some embodiments.

FIGS. 4A-4C illustrate exemplary operational states of an N-typejunctionless transistor, in accordance with some embodiments.

FIGS. 5A-5C are cross-sectional views of various fabrication steps offorming a n-type junctionless transistor, in accordance with someembodiments.

FIGS. 5D-5F are isometric views of various fabrication steps of forminga n-type junctionless transistor, in accordance with some embodiments.

FIG. 6 is an isometric view of a p-type junctionless transistorstructure, in accordance with some embodiments.

FIG. 7 is a top view of a nonvolatile memory array, in accordance withsome embodiments.

FIGS. 8A-8F are cross-sectional views illustrating exemplary structuresat intermediate stages of fabrication a 3D cross-bar nonvolatile memoryarray, in accordance with some embodiments.

FIG. 9 is a flow diagram illustrating a method, according to someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features are disposed between the first and second features,such that the first and second features are not in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition does not in itselfdictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The acronym “FET,” as used herein, refers to a field effect transistor.A very common type of FET is referred to as a metal oxide semiconductorfield effect transistor (MOSFET). Historically, MOSFETs have been planarstructures built in and on the planar surface of a substrate such as asemiconductor wafer. But recent advances in semiconductor manufacturinghave resulted in the use vertical structures.

The terms “S/D” and “source and drain” when used in the context of aMOSFET refer to the source and drain junctions that form two of the fourterminals of a FET.

The terms “S/D” and “source and drain” when used in the context of ajunctionless FET refer to the source and drain terminals that form twoof the three terminals of a junctionless FET in accordance with someembodiments.

The term “junctionless transistor” refers to a transistor architecturehaving a first S/D terminal, a second S/D terminal, and a channeldisposed between the first and second S/D terminals. The junctionlesstransistor's channel has a high doping concentration and is of the sameconductivity type as the first and second S/D terminals. Junctionlesstransistor is referred to herein as a “JLFET.” In some embodiments,ultra-high doping is a doping concentration greater than or equal to5×10¹⁸ atoms/cm³. A JLFET is referred to an n-type JLFET when its firstand second S/D terminals and channel are all n-type. Likewise, a JLFETis referred to as a p-type JLFET when its first and second S/Dterminals, and channel are all p-type. JLFETs further include a gateelectrode disposed over the channel and separated therefrom by a gatedielectric.

The expression “crystalline layer” herein refers to a layer or structureof single crystal material. Likewise, the expression “epitaxially-grown”herein refers to a layer or structure of single crystal material.Epitaxially-grown material may be doped or undoped.

The term “vertical,” as used herein, means nominally perpendicular tothe surface of a substrate.

Various embodiments in accordance with this disclosure provide methodsof making an integrated circuit having a 3D cross-bar nonvolatilememory. Methods in accordance with some embodiments incorporatecrystalline JLFETs into the 3D cross-bar nonvolatile memory within arelatively low thermal budget. Specifically, a layer of dopedcrystalline semiconductor material is transferred from a seed wafer toform the source, drain, and connecting channel of a JLFET. Other methodsof making 3D nonvolatile memory use of a high-temperature annealing stepto crystallize doped source and drain regions after the doping process.The conventional annealing process is either a solid-phasecrystallization anneal for a few hours at 600° C., or a short anneal fora few nanoseconds at high temperature (e.g., 1,100° C.). These processesrequire a high thermal budget. One benefit of using transferred dopedcrystalline layers to form JLFETs is that an annealing process caneither be performed on the JLFET structure prior to transferring on tothe substrate, or be eliminated by directly incorporating dopedcrystalline semiconductor layers into the JLFET structure.

Before describing the embodiments related to the design and fabricationof 3D monolithic cross-bar nonvolatile memory structures, an embodimentoperation process for a JLFET is presented.

FIG. 1 illustrates an isometric view of a JLFET 1 according to thedisclosure. A semiconductor nanowire structure, for example a nanowire,fin, or nanoribbon, forms a source 104 s, a drain 104 _(D), and achannel (covered by gate dielectric and not visible in FIG. 1) partiallysurrounded by a gate electrode 108. Source 104 _(S) and drain 104 _(D)are portions of the semiconductor nanowire that are not surrounded bygate electrode 108. A gate dielectric 106 is disposed between gateelectrode 108 and the channel. Therefore the channel is covered by gateelectrode 108 and gate dielectric 106, and is not visible in FIG. 1.

In a conventional MOSFET the S/D junctions are self-aligned to the gatestructure. In a similar way, the S/D terminals of a JLFET areself-aligned to the gate structure of the JLFET.

FIG. 2 shows an embodiment of n-type JLFET. A first source/drainterminal 204 _(S), a channel 204 _(C) and a second source/drain terminal204 _(D) are patterned from an n-type crystalline silicon semiconductormaterial. Gate electrode 208 is p-doped polysilicon. A gate dielectric206 is disposed between gate electrode 208 and channel 204 _(C).

FIG. 3 shows an embodiment of p-type JLFET. A first source/drainterminal 304 _(S), a channel 304 _(C) and a second source/drain terminal304 _(D) are patterned from a p-type crystalline silicon semiconductormaterial. Gate electrode 308 is n-type polysilicon. A gate dielectric306 is disposed between gate electrode 308 and channel 304 _(C).

FIGS. 4A-4C are a number of views illustrating a sample operation of ann-type JLFET in accordance with some embodiments. The operation of thedevice (example of an n-type device) with various gate voltagesV_(G)(A)<V_(G)(B)<V_(G)(C) is as follows:

As shown in FIG. 4A, for a low gate voltage, for example 0V, channelregion 204 _(C) under gate electrode 208 is depleted of carriers and nocurrent can flow between source 204 _(S) and drain 204 _(D). The deviceis effectively in an OFF state.

As shown in FIG. 4B, for a higher gate voltage, for example 0.4V,channel region 204 _(C) under gate electrode 208 is partially depletedof carriers and some current can flow between source 204 _(S) and drain204 _(D).

As shown in FIG. 4C, for a still higher gate voltage, for example IV,the region under gate electrode 208 is no longer depleted of carriersand can flow between source 204 _(S) and drain 204 _(D). The device isan ON state.

It will be appreciated that current can be further increased if the gatevoltage is increased beyond V_(G)(C) through an increase of the electronconcentration in the region underneath the gate.

FIGS. 5A-5F provide various views of a semiconductor device thatincludes JLFETs during various stages of fabrication. The fabricationprocess provided here is exemplary, and many other steps may beperformed that are not shown in these figures.

As illustrated in FIG. 5A, the fabrication process starts with a seedwafer 500 and a device wafer 510. Wafer 500 includes a first substrate502 and a crystalline semiconductor layer 504′. Wafer 500 may alsoinclude other suitable layers, such as other dielectric layers orimplanted layers. The other suitable layers may be placed between firstsubstrate 502 and crystalline semiconductor layer 504′, or embeddedwithin first substrate 502. First substrate 502 is used as a seed waferfor mechanically supporting crystalline semiconductor layer 504′, andmay comprise any suitable material, for example, silicon. In someembodiments, crystalline semiconductor layer 504′ is a silicon-basedmaterial. For example, crystalline semiconductor layer 504′ is comprisedof crystalline silicon and may be of various different crystalorientations, e.g., having a (100), (110), or (111) crystal orientation.In an embodiment, crystalline semiconductor layer 504′ is directlyformed over first substrate 502 through an epitaxial growth process. Forexample, crystalline semiconductor layer 504′ may be epitaxially grownsilicon or silicon germanium. In another embodiment, the crystallinestructure of crystalline semiconductor layer 504′ is obtained through asolid-phase epitaxial (SPE) regrowth method. In another embodiment, thecrystalline structure of crystalline semiconductor layer 504′ isobtained through ion implantation and annealing, or any other dopingtechnique. The top surface of crystalline semiconductor layer 504′ maybe covered by an oxide layer, such as silicon dioxide layer (not shownin the figures).

In an embodiment, crystalline semiconductor layer 504′ is a dopedsemiconductor layer. Crystalline semiconductor layer 504′ can be ann-type doped silicon layer that is doped with phosphorus (Si:P) or withboth phosphorus and carbon (Si:CP). Carbon could impede theout-diffusion of phosphorus from silicon-based material. In someembodiments, crystalline semiconductor layer 504′ can be an n-type dopedsilicon layer that is doped with arsenic. Other types of dopants mayalso be included. In some embodiments, the phosphorus dopantconcentration is in a range from about 5×10¹⁸ atoms/cm³ to about 5×10¹⁹atoms/cm³. In some embodiments, the carbon dopant concentration is in arange from about 0% to about 5% (atomic percent). Crystallinesemiconductor layer 504′ may also be a p-type heavily-doped siliconlayer. For example, crystalline semiconductor layer 504′ may be heavilydoped with boron. Other types of dopants for forming p-type dopedsilicon layer may also be included, for example, gallium or indium. Ionimplantation has been used as a doping process for many technologynodes. Embodiments in accordance with the present disclosure are notlimited to ion implantation as the doping process for crystallinesemiconductor layer 504′. An annealing process may be performedsubsequent to the doping process.

Device wafer 510 includes a second substrate 512 and an isolation layer514. Second substrate 512 may be a silicon substrate. Alternatively,second substrate 512 may comprise another elementary semiconductormaterial, such as germanium; a compound semiconductor including siliconcarbide, gallium arsenide, gallium phosphide, indium phosphide, indiumarsenide, and/or indium antimonide; an alloy semiconductor includingSiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; orcombinations thereof. Second substrate 512 may also be an n-type orp-type doped silicon layer. The second substrate may be a processedintegrated circuit wafer containing e.g., a plurality of transistorsconfigured to be CMOS circuits. These circuits may include logic,analog, RF (radio-frequency) parts made out of a variety of transistors,capacitors, resistors and interconnections. Isolation layer 514 may bean inter-layer dielectric (ILD)/inter-metallization dielectric (IMD)layer. Isolation layer 514 comprises, for example, a dielectricmaterial, such as phosphosilicate glass (PSG), borophosphosilicate glass(BPSG), FSG, SiOxCy, Spin-On-Glass, Spin-On-Polymers, silicon carbonmaterial, compounds thereof, composites thereof, combinations thereof,or the like, by any suitable method known in the art, such as spinning,chemical vapor deposition (CVD), and plasma-enhanced CVD (PECVD). Itshould also be noted that isolation layer 514 may comprise a pluralityof dielectric layers that are embedded with metallic interconnects, suchas copper interconnects and tungsten, cobalt or titanium nitride vias.

Device wafer 510 may also include a logic circuit, a CMOS circuit, ananalog-to-digital converter, a data processing circuit, a memorycircuit, a bias circuit, a reference circuit, and the like.

FIG. 5B illustrates a bonding process performed to bond seed wafer 500and device wafer 510. With crystalline semiconductor layer 504′ fromseed wafer 500 and isolation layer 514 from device wafer 510 facing eachother, seed wafer 500 and device wafer 510 are bonded using, forexample, a direct bonding process such as dielectric-to-dielectricbonding (e.g., oxide-to-oxide bonding), metal-to-dielectric bonding(e.g., oxide-to-copper bonding), any combinations thereof and/or thelike. The bonding occurs between the top surface of seed wafer 500,i.e., the surface of crystalline semiconductor layer 504′, to the topsurface of device wafer 510, i.e., the surface of isolation layer 514.Prior to bonding, the surfaces of the wafers to be bonded are cleaned toremove any residual liquids or particles from the wafer surfaces. Thebonding process forms a wafer assembly 520.

The bonding may be at wafer level, wherein seed wafer 500 and devicewafer 510 are bonded together, and are then separated. Alternatively,the bonding may be performed at the die-to-die level, or thedie-to-wafer level.

Referring to FIG. 5C, a thinning process is performed to remove firstsubstrate 502 from wafer assembly 520. The thinning process isimplemented by using suitable techniques such as grinding andchemical-mechanical polishing (CMP). Besides thinning process, bondingand separation processes are also required. As a result of the thinningprocess, first substrate 502 is removed or detached and crystallinesemiconductor layer 504′ is exposed.

Crystalline semiconductor layer 504′ is further processed to formnanowires, fins, or nanoribbons (hereinafter referred to as“nanowires”). 504. Nanowires 504 are used as the source/drain andchannel regions of JLFETs. The nanowires are photolithographicallypatterned from crystalline semiconductor layer 504′ using well-knownprocess operations including, but not limited to, forming and patterninga resist layer, etching the exposed portions, and stripping thepatterned resist. Although FIG. 5C shows nanowires 504 to have arectangular cross-sectional area, nanowires 504 can be formed into anysuitable shapes.

Referring to FIGS. 5D-5F, isometric views of various intermediatestructures that result from an embodiment process of making an n-typeJLFET are shown. An n-type JLFET can be fabricated using the previouslydescribed wafer assembly 520. In this exemplary embodiment, nanowires504 are doped n-type crystalline silicon material, isolation layer 514comprises an ILD layer embedded with copper interconnects, and secondsubstrate 512 comprises doped p-type silicon material. Second substrate512 and isolation layer 514 may include a logic circuit, a CMOS circuit,an analog-to-digital converter, a data processing circuit, a memorycircuit, a memory control circuit, a bias circuit, a reference circuit,and the like. In some embodiments wafer assembly 520 includes aninsulator layer positioned at the interface between nanowires 504 andisolation layer 514 (not shown in FIGS. 5D-5F).

In FIG. 5E, a gate dielectric layer 206′ is formed around at least aportion of nanowire 504. In an embodiment, gate dielectric layer 206′ isinitially deposited on the exposed surfaces of nanowire 504 andisolation layer 514. For a charge-trapping nonvolatile memory array,gate dielectric layer 206′ is a stack of materials such as, but notlimited to, oxide-nitride-oxide (ONO). ONO stack is reliable oversilicon surfaces, and is typically used as a capacitor insulator. ONOstack can be formed by thermally oxidizing a silicon surface to form anultra-thin bottom oxide layer, depositing an LPCVD silicon nitridelayer, and oxidizing the silicon nitride layer to form a top oxidelayer. Other materials, such as NO, Ta₂O₅, TiO₂, lead zirconate titanate(PZT), or barium strontium (BST) can be used as the gate dielectriclayer 206′.

As illustrated in FIG. 5F, one or more gate electrodes 208 are formedaround a portion of gate dielectric layer 206′ to form n-type JLFETs.The portions of gate dielectric layer 206′ that are in contact with gateelectrodes 208 form a plurality of gate dielectrics 206 which are notvisible in FIG. 6 since they are covered by gate electrodes 208. Gatedielectric 206 and gate electrodes 208 together comprise gate regionsthat are configured to control conductivity of the channel regionsconnecting the source/drain regions. Gate electrode 208 may be formedfrom any suitable metal or electrically conductive material, forexample, TiN, Pt, Ni, silicide, doped p-type silicon material, or othermaterial/combinations thereof. Gate electrode 208 may be formed using alitho-etch gate-first process or a damascene process. In an embodiment,gate dielectric layer 206′ can be further processed to remain only undergate electrodes 208, such that the source/drain areas would be exposedfor further processing. The n-type JLFET thus includes source/drainregions, channel region, and gate region including gate dielectric 206and gate electrode 208. FIG. 5F shows a plurality of JLFETs connected inseries with each of the plurality of JLFETs having a pair of S/Dterminals.

In FIG. 6, a p-type JLFET is formed using a similar process as describedwith respect to FIGS. 5A-5F. In this exemplary embodiment, nanowires 504are heavily doped with p-type dopants, such as boron, gallium or indium.Second substrate 512 may also be a silicon layer that is doped withn-type, p-type, or H-type dopants. Second substrate 512 may also be anun-doped silicon layer. Further, second substrate 512 may be a processedintegrated circuit wafer containing e.g., CMOS circuits. These circuitsmay include logic, analog, RF (radio-frequency) parts made out of avariety of transistors, capacitors, resistors and interconnections. Thep-type JLFET may thus include source/drain regions, channel region, andgate regions having gate dielectric 306 and gate electrode 308. Sincegate dielectric 306 is covered by gate electrode 308, only gatedielectric layer 306′ is visible in FIG. 6.

The JLFETs described above with respect to FIGS. 1-6 may be used to forma variety of structures, such as but not limited to, 3D nonvolatilememory arrays, described below with respect to FIGS. 7-8F.

FIG. 7 shows a top view of a nonvolatile charge-trapping memory array700 that includes JLFETs. In this exemplary embodiment, nanowires 504and electrodes 108 are used as bit lines and word lines, respectively,to form nonvolatile memory arrays having a cross-bar architecture. As aresult, memory cells of nonvolatile memory 700 are located at theintersections of word lines and bit lines, allowing the cells to beaddressed individually. Gate dielectrics 106 (not shown in FIG. 7) arealso formed at each JLFET. Gate dielectrics 106 may be a suitablecharge-trapping stack of materials, such as oxide-nitride-oxide (ONO).Nonvolatile memory array 700 may be formed on device wafer 510 (notshown in FIG. 7) as explained in detail below.

FIG. 8A depicts a cross-sectional view taken along cut 702 of FIG. 7, ofan embodiment device wafer 510 prior to the transfer of crystallinesemiconductor layer 504′. As explained above with reference to FIG. 5A,device wafer 510 may include one or more devices or features formedtherein and/or thereon. An example of such one or more devices orfeatures is illustrated in FIG. 8A, which includes metallicinterconnects 803 and transistors 805 formed in second substrate 512 andisolation layer 514. There may be multiple isolation layers 514′embedded with interconnects 803 and transistors 805. The metallicinterconnects 803, such as copper vias, provide electrical connectionsfor various portions of device wafer 510. In an exemplary embodiment,isolation layer 514 may be an ILD layer embedded with copper vias.

FIG. 8B is a cross-sectional view taken along cut 702 of FIG. 7 showingdevice wafer 510 with a transferred crystalline semiconductor layer 504′formed on the top surface. As described above with reference to FIGS.5A-5C, crystalline semiconductor layer 504′ may be transferred from aseed wafer 500 through a suitable wafer bonding method. For example, adirect bonding process, a metal-to-dielectric bonding process, anycombinations thereof and/or the like. Crystalline semiconductor layer504′ is doped with the desired dopant type and concentration for makingn-type or p-type JLFETs prior to the transferring process.

FIG. 8C, shows crystalline semiconductor layer 504′ after it has beenfurther processed to form nanowires 504. As shown in FIG. 8C, aplurality of nanowires 504 are formed on isolation layer 514. Nanowires504 are used as the source, drain and channel regions of JLFETs.Nanowires 504 are patterned from crystalline semiconductor layer 504′using suitable processes including photolithography and etch processes.The plurality of nanowires 504 are used as bit lines of 3D nonvolatilememory array 700.

In FIG. 8D, similar to the processes described above with reference toFIG. 5F, gate regions, including gate dielectrics 106 (not shown) andgate electrodes 108, are formed over the plurality of nanowires 504. Asshown in FIGS. 7 and 8D, each gate electrode 108 can be used to controla corresponding JLFET. In this exemplary embodiment, each JLFETfunctions as a nonvolatile memory cell. A second isolation layer 804 isshown formed over the plurality of JLFETs. In further embodiments,second isolation layer 804 provides a surface for stacking additionalmemory arrays. Similar to isolation layer 514, second isolation layer804 may be an ILD layer formed of a low-k dielectric material by anysuitable method known in the art, such as spin-on, CVD, and PECVD.Second isolation layer 804 may similarly include a plurality ofdielectric layers embedded with metallic interconnects.

In FIG. 8E, word line interconnects 803 _(W) connecting gate electrodes108 and metallic interconnects 803 are formed in isolation layer 514 andsecond isolation layer 804, providing vertical electrical connectionsbetween nonvolatile memory array 700 and transistors 805 in device wafer510. This 3D cross-bar architecture creates a structure with higherstorage density and smaller device footprint than 2.5 gigabits persquare millimeter. Word line interconnects 803 _(W) may be metallicinterconnects, such as conductive vias or conductive wires, and may alsobe a multilayer interconnection (MLI) including vertical and horizontalinterconnects, such as conventional vias or contacts, and horizontalinterconnects, such as metal lines. The MLI structure may includeconductive lines, conductive vias, and/or interposing dielectric layers(e.g., interlayer dielectric (ILD)). The MLI structure also provideselectrical connection to and between the transistors. The conductivelines in various levels may comprise copper, aluminum, tungsten,tantalum, titanium, nickel, cobalt, metal silicide, metal nitride,polysilicon, combinations thereof, and/or other materials possiblyincluding one or more layers or linings. The linings include adhesionlayer, barrier layer, etch stop layer, and anti-reflective coatings. Theinterposing or inter-layer dielectric layers (e.g., ILD layer(s)) maycomprise silicon dioxide, fluorinated silicon glass (FSG), or at leastone low-k dielectric material. The MLI may be formed by suitableprocesses typical in CMOS fabrication such as, but not limited to, CVD,PVD, ALD, plating, spin-on coating, and/or other processes. In oneexample, a damascene process is used to form a copper multilayerinterconnection structure. In an exemplary damascene process, an openingis formed in a dielectric layer, which separates the vertically spacedmetallization layers. The opening is typically formed using conventionallithographic and etching techniques. After formation, the opening isfilled with titanium nitride, tungsten or other metals, metal alloys, orstacks of metals and/or metal alloys to form a via. Excess metalmaterial on the surface of the dielectric layer is then removed bychemical mechanical polishing (CMP). Copper or conductive material formsinterconnect lines connected to the vias.

FIG. 8F is a cross-sectional view of the exemplary nonvolatile memoryarray 700 and device wafer 510 taken along cut 704 of FIG. 7 after theformation of bit line interconnects 803 _(B). Bit line interconnects 803_(B) also provide vertical electrical connections between nonvolatilememory array 700 and transistors 805 in device wafer 510. Bit-lineinterconnects 803 _(B) may also be metallic interconnects, such as butnot limited to tungsten vias or conductive wires. Various layers of bitline interconnects 803 _(B) can be used to connect the various featuresdescribed above. Bit line interconnects 803 _(B) may also be amultilayer interconnection including vertical and horizontalinterconnects, such as conventional vias or contacts, and horizontalinterconnects, such as metal lines. In one example, a damascene processis used to form copper-based multilayer interconnection structures.

FIG. 9 is a flow diagram of an illustrative method 900 of forming 3Dcross-bar nonvolatile memory array using crystalline JLFETs. Otheroperations may be performed between the various operations of method900.

Method 900 begins with operation 902, providing a substrate having oneor more devices or features formed therein or thereon. The substrate maycomprise multiple layers, for example, an ILD layer, a dielectric layer,or an implanted layer, and may have devices or electrical interconnectsembedded therein. In some embodiments the substrate is a bulk Si waferhaving one or more devices or features formed therein or thereon.

Method 900 continues with operation 904, transferring a semiconductorlayer to the substrate, where the semiconductor layer is a dopedcrystalline semiconductor layer. The crystalline semiconductor layer maybe a portion of a seed wafer or attached to a seed layer. Thecrystalline semiconductor layer is doped, prior to the transferringprocess, with a desired dopant type and a concentration appropriate forn-type or p-type JLFETs. The transferring process may start with bondingthe semiconductor layer to the substrate. A direct bonding process, ametal-to-dielectric bonding process, and any combinations thereof and/orthe like may be used. If the semiconductor layer is attached to a seedlayer or is a portion of a seed wafer, the transferring process maycontinue with removing the seed layer or the remainder of the seedwafer. The removal process may be implemented by using suitabletechniques such as grinding, chemical-mechanical polishing (CMP), aSmart Cut™ procedure, an ELTRAN® procedure, and/or chemical etching. Asa result of the thinning process, the seed layer or the remainder ofseed wafer is removed and crystalline semiconductor layer is transferredto the substrate and exposed for subsequent processing.

Method 900 continues with operation 906, forming nanowires from thedoped crystalline semiconductor layer. The nanowires are subsequentlyused as the source/drain terminals and channel regions JLFETs. Nanowirescan be fabricated from the crystalline semiconductor layer usingsuitable processes including photolithography and etch processes.Nanowires may be patterned into any suitable shapes.

Method 900 continues with operation 908, forming a gate dielectric layerand gate electrodes around portions of the nanowires. The gatedielectric may include a high-k dielectric material, anoxide-nitride-oxide (ONO) stack of material, or other suitable material,and may be formed by ALD, PECVD, and/or other suitable depositionprocesses. Gate electrodes are formed over a portion of the gatedielectric. Gate electrodes and gate dielectrics together comprise gateregions that are configured to control conductivity of the channelregions. Gate electrodes may include any suitable electricallyconductive material and can be formed using a litho-etch gate firstprocess or a damascene process. Crystalline nanowires and gateelectrodes are used as bit lines and word lines, respectively, to formnonvolatile charge-trapping memory arrays having a cross-bararchitecture. As a result, memory cells of the nonvolatile memory arrayare located at the intersections of word lines and bit lines, allowingthe cells to be addressed individually.

Method 900 continues with operation 910, forming interconnects thatprovide electrical connection for the nonvolatile memory arrays.Interconnects comprise bit-line interconnects and word-lineinterconnects, and are formed in ILD layers or substrates to provideelectrical connections between the nonvolatile memory array and othercircuits and power supplies. This 3D cross-bar architecture creates astructure with higher storage density and a smaller footprint than 2.5gigabit per square millimeter. Bit-line and word-line interconnects maybe metallic interconnects, such as vias or conductive wires.

One benefit of embodiments in accordance with this disclosure is thatJLFETs is created without a need for thermal processing. All theoperations described above, including bonding seed wafer 500, removingfirst substrate 502, etching, forming source/drain and channel regions,and forming gate dielectric 106 and gate electrode 108, are performed atlow temperatures, which do not cause damage to device wafer 510 or theformed wafer assembly 520. In an embodiment, all processing steps areperformed at temperatures less than 600° C. Such low temperatureprocessing enables the stacking of several layers of such devices, withthe addition of each layer comprising low temperature processing thatdoes not cause damage to previously formed device layers.

Another benefit of embodiments in accordance with this disclosure isthat other processes that require heating and may be necessary to formdoped crystalline semiconductor layer 504′ such as crystallization, ionimplantation, or annealing, can be performed prior to the bonding andtransferring processes. This prevents devices formed in underlyinglayers (e.g., the one or more devices or features formed in device wafer510) from being damaged by the processing temperatures used infabricating JLFETs with crystalline source, drain, and channel regions.

In one embodiment, a method of forming an array of nonvolatile memorycells provides a substrate having an upper dielectric layer and aplurality of transistors formed below the upper dielectric layer. Amulti-layer interconnect is formed above the transistors. The substratemay be a bulk wafer. A doped crystalline semiconductor layer is disposedover the upper dielectric layer and etched to form a plurality ofnanowires that are horizontally-oriented with respect to the substratesurface. A charge-trapping stack of layers are formed on the pluralityof nanowires. A plurality of gate electrodes are formed on thecharge-trapping stack of layers. A first interconnection is formedbetween a first nanowire of the plurality of nanowires and a firsttransistor of the plurality of transistors. A second interconnection isformed between a first gate structure of the plurality of gatestructures and a second transistor of the plurality of transistors.

In another embodiment, a method of forming a JLFET-based nonvolatilememory provides a substrate having a first dielectric layer and aplurality of transistors formed below the first dielectric layer. Amulti-layer interconnect is formed above the transistors. A dopedcrystalline semiconductor layer is disposed on the first dielectriclayer and etched to form a plurality of doped crystalline semiconductorstructures on the first dielectric layer. A plurality of gate structuresare formed on each one of the plurality of doped crystallinesemiconductor structures. A first electrically conductiveinterconnection is formed between a first doped crystallinesemiconductor structure of the plurality of doped crystallinesemiconductor structures and a first transistor of the plurality oftransistors. A second electrically conductive interconnection is formedbetween a first gate structure of the plurality of gate structures and asecond transistor of the plurality of transistors. Each gate structureof the plurality of gate structures includes a gate electrode.

In a further embodiment, a JLFET-based nonvolatile memory structureincludes a substrate having a first dielectric layer, a plurality oftransistors disposed below the first dielectric layer, and a pluralityof doped crystalline semiconductor structures on the first dielectriclayer. The JLFET-based nonvolatile memory structure further includes amulti-layer interconnect above the transistors and a plurality of gatestructures on each one of the plurality of doped crystallinesemiconductor structures. The structure further includes a firstinterconnect electrically coupled between a first one of the pluralityof doped crystalline semiconductor structures and a first transistor ofthe plurality of transistors, and a second interconnect electricallycoupled between a first gate structure of the plurality of gatestructures and a second transistor of the plurality of transistors.

It is to be appreciated that the Detailed Description section, and notthe Summary or Abstract of the Disclosure sections, is intended to beused to interpret the claims. The Summary and Abstract of the Disclosuresections may set forth one or more but not all exemplary embodimentscontemplated and thus, are not intended to be limiting to the subjoinedclaims.

The foregoing disclosure outlines features of several embodiments sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art will appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art will also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the subjoined claims.

What is claimed is:
 1. A method of forming a semiconductor structure,the method comprising: forming a doped nanowire structure on a substratehaving a plurality of transistors; forming a charge-trapping stack oflayers directly on the doped nanowire structure; forming a gateelectrode on the charge-trapping stack of layers; forming a firstinterconnect between the doped nanowire structure and a first transistorof the plurality of transistors; and forming a second interconnectbetween the gate electrode and a second transistor of the plurality oftransistors.
 2. The method of claim 1, wherein forming the dopednanowire structure comprises: growing a semiconductor layer; disposingthe semiconductor layer on the substrate; and patterning thesemiconductor layer.
 3. The method of claim 2, further comprisingimplanting at least one dopant species into the semiconductor layer toform a heavily-doped semiconductor layer prior to disposing thesemiconductor layer on the substrate.
 4. The method of claim 3, whereinthe heavily-doped semiconductor layer comprises an n-type dopantconcentration between about 5×10¹⁸ atoms/cm³ and about 5×10¹⁹ atoms/cm³.5. The method of claim 3, wherein the heavily-doped semiconductor layercomprises a p-type dopant concentration between about 5×10¹⁸ atoms/cm³and about 5×10¹⁹ atoms/cm³.
 6. The method of claim 2, wherein growingthe semiconductor layer comprises epitaxially growing silicon.
 7. Themethod of claim 2, wherein growing the semiconductor layer comprisesepitaxially growing silicon germanium.
 8. The method of claim 1, whereinforming the first and second interconnects comprises etching respectivefirst and second openings in the substrate.
 9. The method of claim 8,further comprising depositing copper into the first and second openings.10. The method of claim 1, wherein forming the charge-trapping stack oflayers comprises: depositing a first oxide layer on the doped nanowirestructure; depositing a nitride layer on the first oxide layer; anddepositing a second oxide layer on the nitride layer.
 11. A method offorming a semiconductor structure, the method comprising: forming acrystalline semiconductor structure; forming a gate structure directlyon the crystalline semiconductor structure, wherein forming the gatestructure comprises: depositing an oxide-nitride-oxide charge-trappingstack over the crystalline semiconductor structure; depositing a gateelectrode layer over the oxide-nitride-oxide charge-trapping stack; andpatterning the gate electrode layer; forming a first interconnect toelectrically couple the crystalline semiconductor structure to a firstterminal of a first semiconductor device; and forming a secondinterconnect to electrically couple the gate structure to a secondterminal of a second semiconductor device.
 12. The method of claim 11,wherein the crystalline semiconductor structure comprises an n-typedopant concentration between about 5×10¹⁸ atoms/cm³ and about 5×10¹⁹atoms/cm³.
 13. The method of claim 11, wherein the crystallinesemiconductor structure comprises a p-type dopant concentration betweenabout 5×10¹⁸ atoms/cm³ and about 5×10¹⁹ atoms/cm³.
 14. The method ofclaim 11, wherein forming the crystalline semiconductor layer comprises:epitaxially growing a semiconductor layer; disposing the semiconductorlayer on the substrate; and patterning the semiconductor layer.
 15. Themethod of claim 14, further comprising implanting at least one dopantspecies into the semiconductor layer to form a heavily-dopedsemiconductor layer prior to disposing the semiconductor layer on thesubstrate.
 16. A semiconductor structure, comprising: a crystallinesemiconductor structure; a gate structure directly on the crystallinesemiconductor structure, the gate structure comprising: acharge-trapping gate dielectric stack; and an electrically conductivegate electrode on the charge-trapping gate dielectric stack; a firstinterconnect electrically coupling the crystalline semiconductorstructure to a first terminal of a first semiconductor device; and asecond interconnect electrically coupling the gate structure to a secondterminal of a second semiconductor device.
 17. The semiconductorstructure of claim 16, wherein the crystalline semiconductor structurecomprises an n-type dopant concentration between about 5×10¹⁸ atoms/cm³and about 5×10¹⁹ atoms/cm³.
 18. The semiconductor structure of claim 16,wherein the crystalline semiconductor structure comprises a p-typedopant concentration between about 5×10¹⁸ atoms/cm³ and about 5×10¹⁹atoms/cm³.
 19. The semiconductor structure of claim 16, wherein theoxide-nitride-oxide layer comprises: a first silicon oxide layer; asilicon nitride layer on the first silicon oxide layer; and a secondsilicon oxide layer on the silicon nitride layer.
 20. The semiconductorstructure of claim 16, wherein the crystalline semiconductor structurecomprises silicon germanium.